Foamed polystyrene articles have been used in a variety of application fields as above due to their high heat insulating property, high water resistance, light weight and adequate mechanical strength. There remains a need for further improving these properties, particularly flame retardancy in each application field. Polystyrene is flammable and generates a large amount of heat upon burning. In addition, polystyrene foams have a large surface area which leads to rapid burn-out once ignited. Thus, polystyrene foams need to be flame-retarded for safety against fire.
Recently environmental concern about fluorinated hydrocarbons generally referred to “FLON” due to the destruction of ozone layer and global warming accelerated replacement of FLONs with flammable hydrocarbons and alkyl ethers as blowing agent for polystyrene foams. This replacement adds some difficulties in the production of flame retarded polystyrene foams. Increased flammability of the foam may be compensated for by increasing the amount of a flame retardant to be added. However, increase in the amount of flame retardant not only adversely affects other requisite properties including mechanical strength and bulk density but increases the production cost.
Usually, polystyrene foams have been flame-retarded with a halogenated flame retardant. Most commonly, hexabromocyclododecane (HBCD) has been used in the extrusion foaming process while tetrabromocyclooctane (TBCO) and HBCD have been used in the bead foaming process.
HBCD and TBCO are relatively thermally unstable and their 5 wt % loss temperature are 230-250° C. and 200° C., respectively. When they are used alone in the extrusion foaming process, they cause various problems including discoloration of the foam. In addition, hydrogen bromide and other decomposition products may corrode parts of the extruding machine. Various attempts have been made in the prior art to use HBCD in combination with a heat stabilizer in the extrusion foaming process. These stabilizers include a combination of an organotin polymer and an isopropylidenediphenyl phosphite compounds (JP59/43060B), a combination of an organotin compound and an alkaline earth metal soap (JP 5/24174B), and hydrotalcite (JP 2004/161868A). Use of organotin compounds in combination with other heat stabilizers may be found in a number of patent literature. These heat stabilitzers present another problem or have been proven to be not satisfactory in performance. The organotin stabilizers are ecologically harmful. The isopropylidenediphenyl phosphite compound is susceptible to hydrolysis. This may prevent recycling of foam scraps. Higher fatty acid alkaline earth metal soaps and hydrotalcite have been proven, in our tests, to be effective to stabilize the brominated flame retardant but they decrease the flame retardancy remarkably.
As a flame retardant other than HBCD and TBCO, tetrabromobisphenol A-bis(2,3-dibromo-2-methylpropyl)ether is known. This compound has a 5 wt % loss temperature of about 270° C. This flame retardant is not satisfactory in terms of thermal stability for use in polystyrene foams particularly in the presence of a foam nucleus agent such as talc, bentonite, kaolin, mica, silica or diatomaceous earth unless some heat stabilizers are combined. JP 51/25061B discloses combined use of the just mentioned flame retardant and a higher fatty acid metal soap. As noted above, the metal soap may remarkably compromise the flame retardancy of the foam although it is effective to stabilize the flame retardant. JP 5/67654B discloses addition of a flame retardant of the above type as a solution in a organic solvent to the polystyrene foam in the absence of any nucleus agent. This method has only limited use and must suffer from discoloring and decreased flame retardancy of the foam where the presence of a nucleus agent is essential to improve the cell texture of the foam.
Other examples of a flame retardant having a 5 wt % loss temperature between 280° C. and 320° C. include tetrabromobisphenol A-bis(2,3-dibromopropyl)ether, tris(2,3-dibromopropyl)isocyanurate and tris(tribromoneopentyl) phosphate. These flame retardants are not capable of imparting the polystyrene foam with the desired level of flame retardancy when used alone and require certain enhancers such as tetrabromobisphenol diallyl ether or tribromophenyl ally ether as disclosed in JP 2003/301064A. The allyl ether compounds tend to compromise the heat stability of the foam when added an amount sufficient to enhance the flame retardancy to the desired level. This deficiency cannot be remedied with the addition of various heat stabilizers.
It has been proposed to add a free radical generator such as 2,3-dimethyl-2,3-diphenylbutane in combination with a brominated flame retardant. See, for example, JP 44/9821B, JP 2003/292664A, JP 2004-278010A and JP 2005/8739A. In case of HBCD, the free radical generator is effective at a relatively small amount of addition to enhance the flame retardancy but deteriorative to preserve the initial properties of resin including molecular weight during the processing in order to recycle the foam scraps. For other flame retardants, larger amounts of the generator are required to enhance the flame retardancy which naturally results in the degradation of the initial properties of the resin.
As discussed above, polystyrene foams may be flame retarded with a brominated flame retardant having a 5 wt % loss temperature between 190° C. and 280° C. even at a small amount of addition but the flame retardant of this type may cause various problems due to its low heat stability. With a flame retardant having a 5 wt % loss temperature higher than 280° C., it is imperative to increase the amount of flame retardant to achieve the desired level of flame retardancy which, in turn, compromises other properties of the foam.